T. Koda

Neutral-to-ionic transition in tetrathiafulvalene-p-chloranil as investigated by optical reflection spectra

Abstract Polarized reflection spectra and their temperature-dependence as well are investigated on tetrathiafulvalene-p-chloranil (TTFue5f8CA) single crystals over the temperature range from 2 K to room temperature. Abrupt changes are observed at Tc = 84.4 ± 0.5 K in the charge transfer and intramolecular excitation bands. These experimental results strongly indicate that the dominant part of the neutral-to-ionic transition in this crystal takes place at a sharply defined critical temperature, in contrary to the recent observation by Torrance et al. [Phys. Rev. Lett. 47, 1747 (1981)] that the transition occurs in a temperature range of about 30 K in width. A simple model is presented to account for the observed shift of the TTF+ intramolecular transition bands at Tc.

Nonlinear optical study of quasi‐one‐dimensional platinum complexes: Two‐photon excitonic resonance effect

Third‐harmonic generation measurements have been made for the first time on oriented films of halogen‐bridged mixed‐valence platinum complexes for the pump wavelength region from 0.6 to 1.0 μm. The ‖χ(3)(−3ω;ω,ω,ω)‖ spectra obtained by the Maker’s fringe method show a distinct peak at about 1.8 eV with the peak value of about 4×10−11 esu. From comparison with the electroabsorption data, this peak has been identified with the two‐photon resonance peak with the forbidden exciton in the Pt‐halogen chains. One‐dimensional exciton states in this compound take an important role in the third‐order optical nonlinearity, similar to the case in conjugated polymers.

Switching effect in organic charge transfer complex crystals

A reproducible current switching effect has been observed in various mixed‐stack organic charge transfer crystals whose ionicities are close to the neutral‐ionic phase boundary. This effect appears to be related to the intrinsic nonlinear conductivity, which has recently been reported in these crystals. This switching has a reasonably large delay time and our results indicate that it is a bulk property of the crystals. Discussion is given on the possible mechanism of this effect.

Excitonic resonant effect in the third-order nonlinear optical properties of blue- and red-form polydiacetylene films

Abstract The third-order nonlinear optical susceptibility |χ(3)| has been evaluated for blue- and red-form polydiacetylene (PDA) oriented films by means of the Maker fringe method. For the red-form PDA, care was taken to minimize the conformation disorder effect. The observed |χ(3)| spectra for the blue- and red-form films show prominent peaks at about 0.65 (blue) and 2.35 eV (red) which are just three-photon resonant to the 1Bu excitons located at 1.95 (blue) and 2.35 eV (red) in the respective forms of PDA. The results are discussed by a three-level model taking account of the results of electro-absorption measurements on the same films.

Photo-and thermo-chromism in vacuum-deposited polydiacetylene films

Abstract The change of absorption spectra by UV light irradiation (photo-chromism) and by thermal annealing (thermo-chromism) have been investigated on vacuum-deposited polydiacetylene (12-8 PDA) films. The results are compared with those of the Langmuir-Blodgett films of the same 12-8 PDA. It is suggested that the blue-to-red colour conversion of these films is associated with a isometric transformation of the polymer backbone structures. The films also show a remarkable color change by a visible laser-beam irradiation. A feasibility of utilizing this effect as photo-chromic recording is suggested.

Electro-reflectance spectra of charge-transfer excitations in copper phthalocyanine single crystals

Abstract Electro-reflectance spectra have been measured on copper phthalocyanine single crystals (β form) at 77 K. Prominent features observed on the high energy side of the molecular excitons were attributed to the charge-transfer (CT) excitons. A model is presented to interprete the experimental results by taking account of the coupling effect between the CT and molecular excitons.

Pressure-induced neutral-to-ionic phase transition in TTF-p-chloranil studied by infrared vibrational spectrocopy

Abstract Pressure-induced neutral-to-ionic (NI) phase transition in TTF-p-chloranil has been investigated by means of infrared vibrational spectroscopy. Below 10 Kbar, the ionicity of molecules increases continuously with increasing pressure, in contrast with the first-order-like behaviour at the thermally induced NI phase transition. At about 11 Kbar, however, the ionicity shows a sharp increase indicating an onset of the first-order transition to the more ionic phase. At the same time, the lattice undergoes a large dimeric distortion as evidenced by a strong enhancement of the normally infrared-inactive (ag) molecular vibration.

Analysis of excitonic resonant effect in the third-order nonlinear optical susceptibility of polydiacetylene films

Abstract The third-order nonlinear optical properties have been investigated on the oriented blue- and red-form films of alkyl-urethane-substituted polydiacetylenes, by means of third harmonic generation (THG) and electro-absorption (EA) measurements. The spectra of |x (3) (−3ω;ω,ω,ω| and Im x (3) (−ω;0,0,ω) have been evaluated from the respective measurements. The results are discussed in terms of a three level model for quasi-one-dimensional excitons on the polymer backbone chains.

X-ray evidence for the molecular dimerization in TTF-chloranil

Discussion sur la possibilite de dimerisation du compose (TTF-CA) a une temperature inferieure a la temperature de transformation neutre-ionique

Optical spectra of monomeric and polymeric diacetylene crystals in the synchrotron radiation region of 1–20 eV

Abstract Polarized reflection spectra have been measured on monomeric and polymeric crystals of bis (p-toluene sulfonate) diacetylene in the synchrotron radiation region of 1–20 eV. Assignments are given for the spectral features observed in both crystals. The one-dimensional Wannier exciton model is proposed to interpret the polymer spectra.